Starting ducks is a very exciting but be cautious! I started with 9 ducks however I wished I would have started smaller. I hatched them myself. If the flock were smaller I might be able to enjoy their individual personalities more. They are all VERY unique and fun to watch! I would say 4-6 is an ideal number. Also being inexperienced it took me awhile to find the best routine/set-up for the ducks and myself. Three things ducks love: Routines, Frozen Peas, and Water (lots and lots of it).
I have 3 jumbo Pekins, 1 crested Pekin, 3 Cayugas, 1 Buff Orpington, 1 Swedish. The Cayugas are defiantly the least sociable. The Buff has an interesting personality but is a tad skiddish. The Pekins seem to be the most sociable in general but it really depends on the individual duck. Luna and Willow are both Pekin hens, however one is shy and the other friendly.
Having an all hen (female) or all drake (male) flock is ideal. However then you deprive them of mating. If you do get a mixed flock keep in mind that over-breeding is a huge problem that can injure/kill the hens. Keeping a 1 drake - 4 hen ratio has worked well for me.
I recommend reading The Ultimate Pet Duck Guidebook by Kimberly Link. An overall guide, its great!